[PATCH] posix_spawn: use a larger min stack for -fstack-check [BZ #21253]

[Florian Weimer]

On 03/16/2017 08:30 AM, Mike Frysinger wrote:
> When glibc is built with -fstack-check, trying to use posix_spawn can
> lead to segfaults due to gcc internally probing stack memory too far.
> The new spawn API will allocate a minimum of 1 page, but the stack
> checking logic might probe a couple of pages.  When it tries to walk
> them, everything falls apart.
>
> The gcc internal docs [1] state the default interval checking is one
> page.  Which means we need two pages (the current one, and the next
> probed).  No target currently defines it larger.

GCC miscomputes the offsets in some cases, so I would not rely on this.

Would it be possible compile the functions involved without 
-fstack-check instead?

>    /* Add a slack area for child's stack.  */
>    size_t argv_size = (argc * sizeof (void *)) + 512;
> -  size_t stack_size = ALIGN_UP (argv_size, GLRO(dl_pagesize));
> +  /* We need at least a few pages in case the compiler's stack checking is
> +     enabled.  In some configs, it is known to use at least 24KiB.  */
> +  size_t stack_size = ALIGN_UP (argv_size, 32 * 1024);

64 KiB pages are common, so this reduces the stack size in many cases.

Thanks,
Florian